In the art, magnetic bearing devices are well known in which a rotor is supported for rotation around a rotation axis by a set of active magnetic bearings. The rotor generally comprises a central shaft which is surrounded by a plurality of active magnetic bearing units, each such unit generally comprising a plurality of electromagnets. Often, the magnetic bearing device further comprises a motor for driving the rotation of the rotor, the motor usually also comprising a plurality of electromagnets.
The shaft generally comprises a plurality of annular targets for the electromagnets which are disposed on the periphery of the shaft. The targets are normally ferromagnetic and serve for closing the magnetic flux paths of the electromagnets. They usually consist of a package of stacked and laminated ferromagnetic metal sheets electrically insulated from each other. Such metal sheet packages minimize the effects of eddy currents, which would lead to strong dissipative losses and to undesired heating of the shaft. The targets are often separated axially by nonmagnetic spacers.
The shaft together with the targets is usually manufactured in a particular manner involving a number of distinctive steps. Normally, first the individual sheets are die-cut (punched) into an annular shape. Then the sheets are stacked into a jacket to form a stacked sheet package. The jacket holds and centers the sheets for grinding the inner diameter to tight tolerances and mounting. Several such sheet packages, separated by annular spacers, are then heated and are slid onto the “naked” rotor shaft (i.e., the inner shaft portion) while hot. Upon cooling, the sheet packages and the annular spacers shrink to yield a radial shrink fit with the central portion of the shaft. Finally, the jacket is removed from the laminated sheet packages.
This procedure can lead to a rather large unbalance of the resulting rotor shaft, mainly because the targets usually slightly change their shape and radial position during shrink-fitting. It is therefore generally necessary to trim the final product by finishing it on a lathe (grinding or turning machine). This additional step is time-consuming and increases the cost of the final product.
The fact that the targets are mounted by a shrink fit leads to a number of additional disadvantages. The shrink fit produces a permanent radial stress load on the target. During rotation, this permanent, static load adds to the dynamic load caused by centrifugal forces. Thereby, the maximum allowable rotational speed is more limited than necessary. A further complication arises from the fact that generally the materials of the inner shaft portion and of the targets have different temperature coefficients. During operation, the rotor shaft generally heats, leading to expansion of these parts to different extents. The shrink fit must be designed in a manner to take this expansion over the whole range of operational temperatures and over the whole range of rotational speeds into account, such that under all operational conditions the total radial stress on the shrink fit neither approaches zero nor becomes too high. This limits the choice of materials for the different parts of the rotor shaft and places very high demands on the dimensional tolerances, especially for small rotors.
In JP-A 01-122333 it has been suggested to fit the targets on the outer periphery of a central shaft portion by axial clamping. However, this concept has not become widely accepted in practice because the stability of a rotor shaft constructed in this way cannot be easily ensured, and large unpredictable variations in the mechanical properties may occur between different rotor shafts constructed in the same manner.
A rotor shaft for a magnetic bearing device often also carries a thrust disk which serves as the target for the electromagnets in the axial bearings. A high rotational speed is often also prevented by the limitations imposed by the presence of this thrust disk. The larger the diameter of the thrust disk, the larger the load on the material of the disk at high rotational speed. Therefore there has been a tendency in the art to keep the thrust disk small, in particular, significantly smaller than the axial bearing units. This, however, entails the use of specifically shaped yokes for the axial bearing units, which guide the magnetic flux radially towards the center and to the air gap with the thrust disk. This, in turn, increases the total length of the magnetic bearing device, hampers the development of smaller axial bearings, and increases costs.